Method of applying carbon nanotube films to plastic substrates

ABSTRACT

A method of forming a film of carbon nanotubes on a substrate includes deposition a solution of electrically charged carbon nanotubes on the surface of a substrate, adsorption of the electrically charged carbon nanotubes onto a surface the substrate of opposite electrical charge through dip coating, using a material with a surface electrical charge opposite to that of the electrically charged carbon nanotubes, and formation of a film of carbon nanotubes on the substrate, wherein the film comprises a plurality of electrically charged nanotubes extending in varying orientations but parallel to a facing surface of the substrate.

STATEMENT OF GOVERNMENT INTEREST

The invention described was made in the performance of official dutiesby one or more employees of the Department of the Navy, and thus, theinvention herein may be manufactured, used or licensed by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

TECHNICAL FIELD

Embodiments of this disclosure relate to a method of forming a carbonnanotube (CNT) film on substrates of different geometry of anelectrically conducting thin film of carbon nanotubes (CNTs), based onelectrostatic adsorption from aqueous nanotube suspensions.Specifically, the disclosure describes a method of deposition of CNTsonto plastic surfaces.

BACKGROUND OF THE DISCLOSURE

A wide range of plastic surfaces can be chemically modified such thatthey acquire a charge when exposed to water through chemicalmodification of their surfaces. These charged surfaces efficientlyadsorb a self-limiting film of CNTs over a period of a few seconds fromaqueous suspensions of the nanotubes. Since chemical modification of thecarbon nanotubes is unnecessary, the intrinsic properties of the tubesremain intact.

CNTs may be deposited onto surfaces by spraying either an aqueous ororganic liquid suspension using an atomizer. Using this technology, theuniformity as well as the surface concentration of the CNTs may bedifficult to control. In addition, the use of a binder is required toirreversibly attach the CNTs to the surface. Further, aerosolization ofCNTs poses potential inhalation health risks. Other deposition methodsapplicable to the preparation of CNT films on plastic substrates havebeen studied, but they either require transfer of a pre-made film orchemical modification of the CNTs, which limits the scope of theirapplication. See, e.g., Burgin, T. P. et al.; Langmuir, 2005, 21 (14),pp 6596-6602; Liu, J. et al.; Chemical Physics Letters, 1999, 303, pp125-129; Swager, T. M. et al.; Advanced Materials, 2008, 20, pp4433-4437; and Gruner, G.; Journal of Materials Chemistry, 2006, 16, pp3533-3539.

In the present disclosure, the method overcomes the above-mentioned CNTfilm deposition drawbacks, in which the ability to prepare conductingfilms provides for applications in smart fabrics, foldable antennas,embedded sensors, and other applications. In addition, the high thermalconductivity of the CNTs may be leveraged to produce fabrics that may beused to cool a soldier's body in situations where it is difficult forthem to maintain their body temperature, i.e., while wearing MOPP(Mission Oriented Protective Posture) gear. The deposited CNT films mayalso be used to render plastic parts conductive for electromagneticshielding and other applications. Further, additional processing of theCNT coated fibers may be used to prepare a variety of plastic basedmaterials, including as conducting plastics.

Accordingly, the present disclosure relates to an improved method, whichincludes forming a thin film of CNTs on the surface of substrates, basedon electrostatic adsorption from aqueous nanotube suspensions.

SUMMARY OF THE DISCLOSURE

In accordance with an embodiment of the disclosure, the method includesa series of steps for forming a thin film of CNTs on the surface ofsubstrates, which include:

-   -   deposition of a solution of electrically charged carbon        nanotubes on the surface of a substrate,    -   adsorption of the electrically charged carbon nanotubes onto the        surface of a substrate of opposite electrical charge through dip        coating, and    -   formation of a film of carbon nanotubes on the substrate,        wherein the film comprises a plurality of electrically charged        nanotubes extending in varying orientations but parallel to a        facing surface of the substrate.

In another embodiment of the disclosure, the method includes forming thefilm on the surface of a plastic substrate.

Other embodiments of the method include adsorbing the solution ontoaminopropyl siloxane functionalized amorphous silicon dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate adsorption of CNTs onto a) aminopropylsiloxane functionalized amorphous silicon dioxide, and b) bare amorphoussilicon dioxide surfaces after soaking in a 0.1 mg/mL suspension ofSWCNTs in 1% aqueous Triton X-100 for 1 minute.

FIGS. 2A and 2B illustrate the adsorption of CNTs onto polystyrene with(a) and without (b) an alkylated poly(ethyleneimine) thin film,according to embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

A more complete appreciation of the disclosure and many of the attendantadvantages will be readily obtained, as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

A method of deposition of CNTs onto the surface of substrates isdescribed, which includes plastic substrates. Since the CNTs areelectrically charged, films may be prepared by adsorption of the CNTsonto a surface of opposite charge through a dip coating or a spray andrinse procedure using a material with a surface charge opposite to thatof the CNTs.

The surface concentration of the CNTs can be controlled by changing theconcentration of the deposition suspension. The upper practical limit isdetermined by the maximum suspension concentration, which depends on thesurfactant being used, typically 0.1-10 mg/mL. The surface concentrationof CNTs deposited in the film presumably relates to the charge densityon the CNTs in a non-linear way, but, this is as yet unconfirmedempirically. Changes in contact times above a few seconds do not affectthe surface concentration. The depositions have been conducted at roomtemperature, and thus effect of temperature on concentration remainsundetermined. This technique has been demonstrated on CNTs from 0.7-9 nmdiameter. As the deposition depends on surface charge, directrelationship to tube diameter is not expected. For highly pure CNTs, thecharge density on the tube is expected to relate to the electricalband-gap of the CNTs. However, all bulk prepared CNTs that were studiedto develop these embodiments have charges or charge densities that aredominated by adsorbed impurities.

In one embodiment, the necessary surface charge for absorption of theCNTs may be generated though modification of the surface with a chemicalmoiety that is ionized under the deposition conditions. In anotherembodiment, the surface functionalization may be achieved either throughchemical modification of the substrate's surface of the native materialor through the deposition of another material, such as a polymer, ontothe surface. This technique has been demonstrated with materials, whichinclude silicon dioxide, through the formation of a self-assembledmonolayer of aminopropylsiloxane (APS) that provides the surface chargenecessary for adsorption of the CNTs. Without functionalization,virtually no CNT adsorption takes place while the APS functionalizedsurfaces rapidly absorb a self-limiting film of CNTs. However, thistechnology is not readily transferable to other materials as the surfacefunctionalization is specific to silicon dioxide.

Turning now to FIGS. 1A and 1B, adsorption of CNTs occurs ontoaminopropyl siloxane functionalized amorphous silicon dioxide (a) aftersoaking in a 0.1 mg/mL suspension of single wall CNTs (SWCNTs) in 1%aqueous Triton X-100 for 1 minute, while bare amorphous silicon dioxidesurfaces (b) show no absorption.

Embodiments of the disclosure may provide a simple means offunctionalizing the surface of plastics with a polymer that generates asurtface charge opposite to the charge on the CNTs, and causes thespontaneous formation of a CNT thin film when exposed to a CNTsuspension. The concept is demonstrated by using an alkylatedpoly(ethyleneimine) (PEI) thin film as the gettering layer for CNTadsorption, as shown by FIGS. 2A and 2B. The PEI film may also becross-linked using a diazide or similar reagent to improve itsmechanical robustness and chemical resistance. In addition, in otherembodiments, the film can be deposited using the same techniques used todeposit the CNT films, including but not limited to dip coating andspraying, enabling a simple, low cost process.

In another embodiment, when the film is deposited, pre-grown nanotubesmay be used. For example, nanotubes may be suspended in a solvent in asoluble or insoluble form, in which arrangement of the film may be oneor more nanotubes thick. The carbon nanotubes may also be deposited bydipping the substrate in a solution of soluble or suspended nanotubes.

Further, the deposition may include forming a patterned film of carbonnanotubes. Specifically, in a preferred embodiment, when the carbonnanotube film is provided over a surface of a substrate, the film ispatterned.

In a typical process, the surface of the substrate is immersed in asolution of about 1 to about 100 mg/mL of alkylated poly(ethylene imine)in hexanol and allowed to dry. The surface is then immersed in asuspension of about 0.1 to about 10 mg/mL CNT in a liquid such as water,rinsed, and allowed to dry. The resulting films may be nearlytransparent, and may be comprised of interconnected CNTs that form atwo-dimensional conducting network.

The polymer films that we have prepared are 10-20 nm thick. Films abovea threshold thickness (unknown) should not change the total amount ofCNTs absorbed. The average thickness of the subsequently adsorbed CNTfilms can range from approximately 1-200 nm.

In view of the above disclosure, the method of the disclosuredemonstrates the transfer of this adsorption methodology to plasticsurfaces. The described method is low cost, simple to implement, andapplicable to a broad range of materials, including but not limited toplastics of different shape. In addition, the films formed from themethod may be used to render plastic parts conductive forelectromagnetic shielding and other applications.

Obviously, numerous modifications and variations of the disclosure arepossible in light of the above disclosure. It is therefore understoodthat within the scope of the appended claims, the disclosure may bepracticed otherwise than as specifically described herein.

What is claimed is:
 1. A method of forming a film of carbon nanotubes ona polymer substrate, comprising: preparing a solution of carbonnanotubes having a first electrical charge, depositing the solution on asurface of the substrate, the surface having a second electrical chargeopposite to the first electrical charge, wherein the carbon nanotubesadsorb onto the surface of the substrate, and forming a film of carbonnanotubes on the substrate, wherein the film comprises a plurality ofelectrically charged nanotubes extending in varying orientationsparallel to the surface of the substrate.
 2. The method according toclaim 1, wherein the plastic substrate is alkylated poly(ethyleneimine).3. The method according to claim 1, wherein the depositing comprises oneof dip coating, spraying, painting, and wiping.
 4. The method accordingto claim 1, wherein the surface of the substrate is immersed in asolution of about 1 to about 100 mg/mL of alkylated poly(ethylene imine)in hexanol and dried.
 5. The method according to claim 1, wherein thefilm comprises interconnected carbon nanotubes that form atwo-dimensional conducting network.
 6. The method according to claim 1,wherein the film has controlled density.
 7. The method according toclaim 1, wherein the film is substantially a monolayer of carbonnanotubes.
 8. The method according to claim 1, wherein the depositingforms a patterned film of carbon nanotubes.
 9. The method according toclaim 1, wherein the substrate is any geometrical shape.